This invention relates to optical fibres and optical fibre devices such as, for example, optical fibre lasers.
Optical fibre lasers for continuous-wave (cw) or pulsed operation make use of amplifying optical fibres arranged with reflectors to form a laser cavity. For example, publication reference [1] describes a single mode Q-switched optical fibre laser system employing a low numerical aperture erbium-doped single mode fibre pumped by a diode laser to give 160 xcexcJ, 50 nS pulses tunable between 1530 and 1560 nm.
Previously, much of the work done on erbium-doped fibres has concentrated on maximising the small signal optical gain, which in turn requires a small xe2x80x9cspot sizexe2x80x9d or mode-field diameter (MFD). This also provides single mode operation considered desirable in applications requiring a high beam quality, communication applications and applications requiring very short pulsesxe2x80x94see [1].
However, a problem which has been noted in such doped fibre devices is that nonlinearity within the core can distort the optical output at high powers, resulting in limits placed on the peak power of pulses which can be accommodated in the fibre before nonlinear distortions such as self phase modulation become apparent. In one example, the maximum tolerable peak power in 1 m of a previous doped optical fibre is about 500 W.
Similar problems can also occur in cw lasers and amplifiers where nonlinear effects such as Brillouin scattering can limit the output power when operating with narrow linewidths (e.g.  less than 10 MHz). For 1 m of conventional fibre in cw operation the nonlinear threshold for Brillouin scattering is about 20 W.
A further restriction on the available output power from pulsed fibre lasers is the energy storage capacity of the amplifying fibre. The high gain efficiencies in conventional single mode fibres limit the energy that can be stored to about 10 xcexcJ.
So, there exists a continuing need for larger and larger peak powers and pulse energies while retaining the possibility of single mode operation, but this is limited by nonlinear effects and low energy storage in conventional fibres.
This invention provides an optical fibre having a cladding layer surrounding a core, the cladding layer comprising at least a first, relatively inner generally cylindrical region, a third, relatively outer generally cylindrical region, and a second region disposed between the first and third regions, the second region having a higher refractive index than the first and third regions; and the peak difference in refractive index between the first cladding region and the core being less than about 0.0030.
A fibre according to the invention is capable of operating in a single transverse mode but with a much higher MFD than in conventional single mode fibresxe2x80x94in some prototypes up to 40 xcexcm. In an amplifying or lasing application this can lead to non-linear effects being dramatically reduced and the energy storage capability of the fibres being dramatically increased, allowing single mode pulse energies in prototype devices of 0.5 mJ or, if a slightly multi mode signal is tolerated, up to 0.85 mJ. It is envisaged that the invention provides technology allowing pulse energies in the mJ regime.
In prototype fibres according to the invention, nonlinear thresholds are 20-25 times higher than in conventional fibres, so the power handling capability of the fibre is correspondingly increased.
As well as being appropriate for pulsed applications, fibres according to the invention can provide increased power in cw single frequency lasers, amplifiers and associated devices and can increase nonlinear thresholds within passive devices such as Bragg gratings.
The fibre design is also compatible with cladding pumping techniques (see [1]), so providing corresponding increases in average output power available from such devices.
The cladding refractive index structure defined above provides two main benefits.
Firstly, it gives an increased spot size for the fundamental guided mode. This reduces nonlinear effects by simply providing a larger cross-sectional area over which the light is propagated, so reducing the energy density within the core.
Secondly, it can decrease the fibre bend loss for the fundamental mode (an established problem). In prototype embodiments an improvement in bend loss of between 10 and 40 dB has been observed. For a prototype 21 xcexcm core fibre the macroscopic bend loss for a 30 cm radius bend was found to be less than 0.1 dB/m.
A further feature arises from the small refractive index difference between the core and the cladding, which in turn means that the fibre has a very low numerical aperture (NA)xe2x80x94as low as about 0.06 in some prototype embodiments. The low NA ensures that there are few viable optical propagation modes even for a large core area, and so can alleviate the problem of coupling of energy (e.g. by amplified spontaneous emission or ASE) into unwanted modes. A preferred large outer diameter of the fibre (e.g. greater than about 200 xcexcm) can also help to alleviate mode coupling.
The arrangement defined by the invention can be highly advantageous when implemented as a single mode fibre, because the low NA and novel cladding structure can spread the fundamental mode beyond the normal bounds of the core and out towards the preferred xe2x80x9cringxe2x80x9d structure within the cladding. This increases the MFD of the fibre, increasing its energy storage capacity and decreasing nonlinear effects, because the energy density at any position is reduced. However, even greater benefits can be obtained in a multimode fibre, i.e. one capable of supporting more than just the fundamental mode (see Appendix one for an analytical derivation of the term xe2x80x9csingle-modexe2x80x9d, although a working definition is widely accepted within the art). In such a case, the MFD can be increased still further, while the low NA acts to restrict the available modes of the structure. Furthermore, in an amplifier or laser configuration, if an amplifying dopant distribution is chosen (such as doping a central region of the core) which overlaps more favourably with one mode (e.g. the fundamental mode, but it could be another mode), the multimode fibre can operate effectively in a single mode. So, the double benefit can be obtained of a fibre having a relatively large xe2x80x9cmultimodexe2x80x9d corexe2x80x94so that the power handling capacity of the fibre core is improvedxe2x80x94operating in a single mode by the influence of the placement of the dopant.
The single mode operation amplifying applications, where the amplifying dopant is preferably substantially confined to the core, arises because the modal overlap of the fundamental mode with the symmetrically doped core is far higher than the modal overlap of any other (higher order) mode. This leads to a significant gain difference between the fundamental mode and other modes, in effect providing single mode operation with a fibre having a large enough core to support multimode operation. (In other embodiments another dopant distributionxe2x80x94perhaps an asymmetric onexe2x80x94could be used so as to favour a mode other than the fundamental).
This invention also provides an optical fibre amplifier comprising a doped fibre as defined above; and means for injecting pump radiation into the fibre.
This invention also provides an optical fibre laser comprising: an optical fibre amplifier as defined above; and reflector means disposed relative to the optical fibre amplifier so as to promote lasing operation within the optical fibre amplifier.